Hello everyone. I have a 91 z28 305 TPI auto. I have accel intake base and runners, ported plenum, MSD distributor, MSD coil, edelbrock shorty headers, flowmaster muffler, and 1.5:1 roller rockers. I also have a chip made from John Wayne.

I was just wondering what type of gas I should be using?

I have been using 93 octane and I want to know if I can go down to 87 octane?

Thank you

 

You could. But why would you want to? Most of the 93 oct has a detergent in it which will help the engine (IMO) in the long run. Yes, there is a price difference. But what are we looking at? $3-$4 for a 15 gallon fill up?

 

The main reason to run 92 and higher octane fuel is to keep the engine from getting spark knock (pinging), which in turn keeps the ECM from dialing out ignition timing, which in turn hurts "performance."

 

It all depends on the timing curve in that "John Wayne" chip.
Try 87 and see if it pings. It may be ok in cold weather, but iffy in the summer, depending where you live and the type of driving.

 

To really know if it's worth going to 87, you need a scan tool to record spark knock while under load. If the ECM detects too much knock, it will retard the timing until knock is reduced. Reduced timing will mean less power so someone who claims they make better power with a higher octane fuel just isn't realizing that the ECM is causing the lack of power with the lower octane. If the ECM doesn't detect knock or very little while under load with 87 octane, putting in a higher octane fuel will not make any more power.

There is the same amount of BTU energy in 87, 92 or 96 octane fuel. The octane rating is the fuels resistance to preignite which causes the knock. If the octane is too low under high load conditions or while using a power adder, spark knock can eventually cause a lot of damage. If the fuel isn't preigniting, the engine can make more power because it will use most of the burnt fuel as direct energy to move the vehicle.

If you're daily driving the vehicle without pushing it hard all the time, 87 octane will probably be just fine. Normal driving with low octane fuel will probably not cause enough knock to be an issue. It's only noticeable when the engine is put under a heavier load, high boost etc.